Even though current technologies of fiber laser have made significant progress toward achieving a compact and reliable fiber laser system providing high quality output laser with ever increasing output energy, however there are still technical difficulties in implementing an air core photonics band gap fiber (PBF) in a fiber compressor. For better understanding the function performed by a pulse compressor, a short pulse high energy fiber laser system as that shown in FIG. 1 is first described. Referring to FIG. 1 for a schematic diagram of a short pulse high-energy fiber laser system that includes a laser seed 15 having an oscillator for generating a fiber-based mode-locking laser with original pulse duration. The laser project from the oscillator of the seed laser 15 is projected into a single mode fiber (SMF) laser stretcher 20 to stretch the laser pulse. The stretcher 20 generates laser pulse with stretched pulse width is projected into a series of laser amplifiers 25 to amplify the laser into higher energy. The amplified laser is then projected into a pulse compressor 30 to re-compress the pulse width of the laser to output a laser with original pulse width. In order to provide compact and reliable fiber laser system, instead using grating lenses for pulse compression as that commonly used in a conventional system, it is desirable and very promising according to test results to implement a compressor with the air core photonics band gap (PBG) fiber. Specifically, the air core PBG fiber is very useful for high-energy fiber laser/amplifier for compressing the chirped pulses back to a couple of hundreds fs. Even though such configuration enables an all fiber system for generating short pulse-high energy lasers, however, due to a limitation in power handling of the end-face of the photonics band gap (PBG) fiber, the outputted laser is still limited to only tens of micro Joules. Laser output of higher energy is not yet feasible due to this limitation when a laser system is implemented with the PBG fiber
In order to further improve the energy handling, different configurations or processes may be implemented to prevent a surface damage of the end-faces of the PBG fiber. End cap of a piece of coreless fiber or glass can be attached to the PBG fiber to increase the mode area of output beam at the end facet. This will make possible to amplify ps to 100 fs pulse to the level of mJ. However, perfect treatment of the two surfaces of two separated pieces of material (PBG fiber with coreless fiber or glass plate) is difficult.
Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide fiber laser with new PBG fiber covered with end caps or with part of the air core and air holes filled with glass powder or liquid glass. The new and novel FBG fiber of this invention increases the mode area of the fiber to improve the energy absorptions and dissipation processes in laser transmission. With the improved FBG fiber to function as a compressor of the laser system, a short pulse high-energy laser system is enabled to provide laser output power up to millie-Joules (mJ) having ps to 100 fs pulse widths.